Relieving to atmosphere

In 2004, at the B.P. Refinery in Texas City, 15 contract workers were killed and 170 seriously burned. The relief valve opened on the top of a naphtha splitter. The relief valve did not vent to the flare, but to a blowdown stack. The blowdown stack was open to the atmosphere and drained to a sewer. This drain had a loop seal to prevent vapors from blowing into the sewer. 

The bottom level control valve was closed in the field. 

The bottom flow meter was off-zero, and showed a substantial flow. 

The bottom liquid level indicator on the panel indicated a less than full level, even when the bottoms level was above the top level tap. 

The operators, being unaware of the last three facts, filled the tower almost to the top with naphtha and increased heat flow to the reboiler. The relief valve, set for 50 psig, opened as the liquid in the tower swelled up due to the extra heat. The boiling naphtha partially flashed as it flowed from 50 psig in the tower to zero psig in the blowdown stack. 

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The outside surfaces of refrigerated storage vessels are thermally insulated to assist in maintaining the desired storage temperature. Thermal excursions increase product vaporization, raise vessel pressure, and increase the amount of gas relieved to atmosphere orto a relief vent collection system. Fire exposure of a refrigerated vessel can increase product temperature and vessel pressure, possibly exceeding the capacity of relief valves or relief vent collection system, and could result in vessel rupture with major fire or explosion consequences. 

Pressure relief device (PRD) is a safety device to protect equipment or piping from damage due to overpressure or vacuum, but in this chapter, we wiU discuss FRD to provcfit overpressure only. Commonly used pressure relief devices are spring-loaded and pilot-operated pressure relief valve (PRV), rupture disk, or pin-actuated pressure relief device. For equipment, it is installed cither directly on the equipment or at its inlet/outlet piping. When the equipment or piping pressure reaches the pressure relief de vice set pressure, it will open to relieve fluid inside the equipment or piping to avoid overpressure. The relief fluid can be vapor, liquid, or two phase mixture. The relieved fluid is either sent to floic system to be disposed or relieved to atmosphere. 

A typical this type PRV is shown in Figure 1. This type of PRV is not designed to avoid backpressure influence. From Figure 1, it shows that disk holder area exposed to PRV backpressure is greater than its nozzle (orifice) area. Therefore, PRV s backpressure tends to hold the disk down. If PRV s backpressure is constant, PRV s set pressure should be set at the target relieving pressure minus a correction pressure due to its backpressure. If PRV s backpressure is variable, there is possibility fiiat PRV doesn t open, when it should. Therefore, conventional PRV should be used when relief load is relieving to atmosphere or a systnn with constant pressure. 

Ensure vents relieve to a safe location. If vented to atmosphere, ensure proper classification and controlled access... 

The most important safety devices in a production facility are the pressure relief valves, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressures higher than their design pressures. Relief valves must be designed to open rapidly and fully, and be adequately sized to handle the total flow of gas and liquids that could potentially cause an overpressure situation. They relieve the pressure by routing this stream to a safe location where it can be vented to atmosphere or burned. 

Conventional relief valves should only be used where the discharge is routed independently to atmosphere, or if installed in a header system, the back-pressure build-up when the device is relieving must be kept below 10% of the set pressure so the set point is not significantly affected. The set point increases directly with back-pressure. 

The pressure in the relief piping is usually equal to atmospheric pressure as long as no relief valve is relieving. There is a common miscon-... 

If flammable gases are likely to be relieved to the atmosphere, the relief stream either cannot reach a source of ignition or, if ignited, it must bum harmlessly. Dilution of the relief stream to a point below the lower flammability limit is highly recommended. A review paper [231] provides details on solving this general problem. 

Reliability is enhanced by complete absence of gas valves. Maximum safety is achieved without depending upon pressure relief valves or exploston discs. Wide open burner design would relieve any abnormally elevated pressure directly into the boiler which in turn is directly vented to atmosphere. 

A one-liter rocker bomb is chained with 50 g. of IFgi it is pressurized to 120 atm. with CO and rocked in an inclined position for a week. The pressure is then relieved imtil atmospheric pressure is reached, thus removing the COFg and the excess CO. Next, a quartz trap containing some Sb powder and cooled with liquid is connected at the valve oL the autoclave. The system is evacuated for an hour to about 200 mm. The COIF thus distills over and is condensed in the trap. The autoclave can then be refilled with CO without recharging the IFg. 

Back pressure regulators provide steady-state control relief valves provide on-off protection from overpressure or vacuum conditions. When the set pressure threshold is exceeded, the valve opens either to atmosphere or to an auxiliary line (where the fluid may go to a flare or storage tank or even to recycling). Once the vessel pressure drops to a predetermined pressure, the valve reseats. The difference in pressure between when the valve relieves and when the valve reseats is referred to as the blowdown it is typically 2-20% lower than the relief pressure. 

If any employee complains of discomfort, the pressure shall be held to determine if the symptoms are relieved. If, after 5 minutes the discomfort does not disappear, the lock attendant shall gradually reduce the pressure until the employee signals that the discomfort has ceased. If he does not indicate that the discomfort has disappeared, the lock attendant shall reduce the pressure to atmospheric and the employee shall be released from the lock. 

Besides reduce the backpressure influence on PRV s disk, bellows has another usage. It sometimes is used to isolate the guide, spring, and bonnet fiom the relieving fluid. It is important that balanced-bellows PRV s bonnet be vented to atmosphere, since it is part of valve s design basis. 

As you evaluate the above criteria, recall that there were scores of towers in Texas City operating for many decades. These towers relieved to the atmosphere (some of which I designed), and nothing ever happened. Nothing ever happened until that terrible incident in 2004. 

It is shown that solute atoms differing in size from those of the solvent (carbon, in fact) can relieve hydrostatic stresses in a crystal and will thus migrate to the regions where they can relieve the most stress. As a result they will cluster round dislocations forming atmospheres similar to the ionic atmospheres of the Debye- Huckel theory ofeleeti oly tes. The conditions of formation and properties of these atmospheres are examined and the theory is applied to problems of precipitation, creep and the yield point."... 

Examples of such backup features are given for each utility below. In designs where all pressure rehef valves discharge into a closed collection system, because of environmental restrictions, a total failure of one utility deserves more careful consideration since there are no atmospheric releases which would tend to relieve the load on the closed system. 

It should be noted that the above maximum radiant heat density criteria for application to inadvertently ignited atmospheric releases from pressure relief valves or vents are less restrictive than those used for flare design. This results from the fact that flares are continuously ignited, whereas ignition of a relieving PR valve is unlikely. In addition, the area surrounding a flare is open and offers no protection, while within a process unit access to shelter is available. 

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